1. Field of the Invention
This invention relates to the hydrogenation of mixed nitroaromatic compounds.
2. Description of the Prior Art
Phenylene diamines, toluidines, naphthylamines and other aromatic amino compounds are customarily produced by catalytic reduction of the corresponding nitro compounds, employing a variety of catalysts and conditions, including temperature and pressure. In general, a single nitro compound can be reduced to the desired amino compound in a batch or a continuous process. When it is desired to reduce a mixture of nitro compounds, for example, an isomeric mixture or a mixture containing mono and dinitro compounds, the mixture can be hydrogenated before any separation of components is effected.
Among the problems associated with the preparation of aromatic amines from aromatic nitro compounds, particularly in commercial quantities, is the differing reactivities of various nitro compounds under the reduction conditions employed. This problem is more acute in catalytic hydrogenation processes than when a reducing agent such as sodium sulfhydrate or sodium hydrosulfite is employed. The activity of the catalyst is very important and it is difficult to find a single catalyst which can be used efficiently to reduce a wide variety of aromatic nitro compounds. Ring hydrogenation and partial reduction of the nitro group may be achieved with catalysts which are either too active or too inactive. Frequently, undesired by-products are produced, for example, high boiling materials or tars which may be almost unidentifiable.
U.S. Pat. No. 3,213,141 discloses a process for hydrogenating an aromatic dinitro compound or a mixture of isomeric aromatic dinitro compounds, in the presence of a nickel or platinum catalyst, wherein said process liquid dinitro compound is introduced into a reaction vessel containing a mixture of water and the corresponding aromatic diamino compound at such a rate that the nitro compound dissolves in the mixture, and a liquid stream is removed from the reaction vessel at such a rate that it is substantially free of unhydrogenated dinitro compound.